The present invention relates generally to integrated circuit device packages, and more specifically to a plastic integrated circuit device package with a lead frame having improved thermal dissipation characteristics.
The trend in the electronics industry towards higher density integrated circuit devices has required that packages which house such higher density integrated circuit devices be able to dissipate more power. Because high density integrated circuit devices require more power and thus generate more heat, the manner in which the integrated circuit device package dissipates heat is critical. Generally speaking, it is quite advantageous to remove the heat from the integrated circuit device to the outside environment as quickly as possible.
The critical thermal path for removing heat from the integrated circuit device is defined as the path: (1) from the integrated circuit die to the die pad on which the integrated circuit die is seated, (2) from the die pad, by way of the horizontal air gap between the lead fingers and die pad, to the lead fingers of the lead frame, and (3) from the lead fingers to the printed circuit board on which the integrated circuit device is seated. Shortening this thermal path improves the thermal dissipation characteristics of the integrated circuit device package. As integrated circuit devices become more dense and thus must dissipate higher power, there is a continual need in the art to improve the heat dissipation characteristics of integrated circuit devices by shortening the thermal path of integrated circuit device packages.
A lead frame is the backbone of a molded plastic package. Lead frames are described in Chapter 8 of the 1989 edition of the Microelectronics Packaging Handbook (available from Van Nostrand Reinhold, 115 Fifth Avenue, New York, N.Y. 10003). In general, a lead frame is fabricated from a strip of sheet metal by stamping or chemical milling. The lead frame serves first as a holding fixture during the assembly process, then, after molding, becomes an integral part of the package. A lead frame includes a plurality of finger-like connections that extend from the periphery of the lead frame toward a center die pad. A semiconductor or chip is mounted on the center die pad.
Lead frames are either chemically milled or mechanically stamped from rolled strip stock. Typical strip thickness is approximately 0.25 mm, with thinner material (of approximately 0.20 mm) used for high lead-count packages such as 84-pin PLCC and quad flat pacs. Chemical milling is a process that uses photolithography and metal-dissolving chemicals to etch a pattern from a metal strip.
Stamped lead frames are fabricated by mechanically removing metal from strip stock with tools called progressive dies. The energy required to shear metal is directly proportional to the length of shear. Lead frames have large shear lengths per unit area. Therefore, a large amount of energy is required to stamp a full frame with one press stroke. Progressive dies are usually made of tungsten carbide and are arranged in stations. Each station punches a small area of metal from the strip as it moves through the die set.
To allow for the cutting tool, also known as a punch die, to be strong enough to operatively cut the lead frame, the prior art uses a cutting tool that has a narrow width of approximately 0.2 mil at the end increasing in width to a maximum width of approximately 30 to 40 mil at the base.
Referring to FIGS. 1a to 7b, the manufacturing process for fabricating a conventional lead frame 10 of a plastic integrated circuit package according to the prior art is illustrated. Fabrication begins with a rectangular sheet of metal from which the plurality lead fingers 12 of the lead frame are formed. Referring to FIG. 1a, the top view of a quadrant of a lead frame 10 after the lead fingers 12 have been defined, including a quadrant of die pad 14, is shown. The plurality of lead fingers 12 are formed from a rectangular sheet of metal as is well known in the art. FIG. 1b illustrates the cross-sectional view of the quadrant of the lead frame 10 at this stage of the process. The next step in the prior art process, as shown in the top view of FIG. 2a, is to clamp the lead frame 10 into a fixed position prior to being cut with a punch die 22. FIG. 2b illustrates the cross-sectional view of the quadrant of the lead frame 10, the upper clamp 18 and lower clamp 20, and the punch die 22. The next step in the prior art process, as shown in the top view of FIG. 3a, is to separate the lead fingers 12 from the die pad 14 of the lead frame. At a substantially central portion of the lead frame 10, a square die pad 14, configured for mounting a semiconductor or chip thereon, supported by a plurality of suspension tie bars 16 is formed by cutting the lead frame 10 with the punch die 22. The punch die 22 having a plurality of recesses along the cutting surface forming the plurality of tie bars 16 as the lead frame 10 is cut. FIG. 3b illustrates the cross-sectional view of the quadrant of the lead frame, the upper clamp 18 and lower clamp 20, and the punch die 22 after the lead frame 10 is cut with the punch die 22. Tie bars 16 connect lead fingers 12 to die pad 14. FIG. 4a is a top view of a quadrant of the lead frame after the lead frame has been cut with a punch die 22 showing the physical separation aim between the lead fingers and the die pad 14. FIG. 4b illustrates the cross-sectional view of the quadrant of the lead frame at this stage of the process. FIG. 5 is a top view of the lead frame after the top and bottom portions of the lead frame have been cut with a punch die 22. FIGS. 6a and 6b illustrate the lead frame showing the physical separation between the lead fingers 12 and the die pad 14. The plurality of lead fingers 12 extend from the periphery of the lead frame 10 to a position spaced apart from the die pad 14 with a predetermined distance represented as xcex94 greater than 0, where xcex94 is defined as the horizontal gap between the lead fingers 12 and the die pad 14. It is also clear that the lead fingers 12 and the die pad 14 are co-planar at this stage of the fabrication process. Referring to FIGS. 7a and 7b, the last step of the process is to downset the die pad 14 in relation to the lead fingers 12. In performing the downset, it is noted that the physical separation between the lead fingers 12 and the die pad 14, represented as xcex94 greater than 0, is maintained. Additionally, the downset of die pad 14 results in a vertical separation between lead fingers 12 and die pad 14.
Referring to FIG. 8, the fabrication of the lead frame of a plastic integrated circuit package, according to the prior art, is illustrated in process flow 30. First, the lead frame begins as a flat metal sheet as shown in step 32. Next, at step 34, the lead fingers 12 are defined. Step 34 corresponds to FIGS. 1a and 1b. After the lead fingers 12 are defined, they are separated from the die pad 14 in step 36, Step 36 corresponds to FIGS. 3a and 3b. Finally, at step 38, the die pad 14 is downset with respect to the lead fingers 12 as illustrated in FIGS. 7a and 7b. 
According to the lead frame formed in FIGS. 1-8, the critical thermal path by which heat must be dissipated is defined as the distance from the integrated circuit die to the downset die pad 14 on which the integrated circuit die is placed; from the die pad 14, by way of the horizontal air gap xcex94 between the lead fingers 12 and die pad 14, to lead fingers 12; and from lead fingers 12 to the printed circuit board on which the integrated circuit device is placed. Shortening this thermal path would improve the thermal dissipation characteristics of the integrated circuit device. There is therefore an unmet need in the art to shorten the critical thermal path of the prior art lead frame used in plastic integrated circuit device packages.
It is therefore an object of the present invention to shorten the critical thermal path of the prior art lead frame used in plastic integrated circuit device packages.
Therefore, according to a preferred embodiment of the present invention, a process for fabricating a lead frame of a plastic integrated circuit package is disclosed. Fabrication begins with a rectangular sheet of metal from which the plurality lead fingers of the lead frame are formed as is well known in the art. Next, the lead frame is clamped into a fixed position. Finally, the die pad of the lead frame is simultaneously separated and downset from the lead fingers of the lead frame by shearing the lead frame with a punch die pair. At a substantially central portion of the lead frame, a square die pad, configured for mounting a semiconductor or chip thereon, supported by a plurality suspension tie bars is formed by shearing the lead frame with a punch die pair and a lower clamp that are mated such that the punch die pair may be inserted into the lower clamp with essentially a negligible gap of no more than 2 percent of the lead frame thickness. The punch die pair having 90 degree cutting surfaces and a plurality of recesses along the cutting surfaces forming the plurality tie bars as the lead frame is sheared. Tie bars connect lead fingers to the die pad. Performing the separation and downset of the die pad from the lead fingers results in essentially no horizontal gap between the lead fingers and the die pad. However, the downset of the die pad with respect to the lead fingers does result in a vertical separation between the die pad and the lead fingers that was also seen in the prior art. The separation and downset step may be accomplished by a simultaneous cutting and pressing operation resulting in the lead frame being sheared.
The lead frame of the preferred embodiment of the present invention has a shorter critical thermal path than the prior art lead frame since there is essentially no horizontal gap between the lead fingers and the die pad of the lead frame, unlike the prior art lead frame. The shorter critical thermal path means that the lead frame is much more efficient at dissipating the heat generated by high density integrated circuit devices.
According to an alternate embodiment of the present invention, the step of simultaneously separating and downsetting the die pad with respect to the lead fingers of the lead frame may be separated into two steps. First, the lead fingers are separated from the die pad using a cutting tool, such as a laser, that results in essentially no horizontal gap. Second, the die pad is downset with respect to the lead fingers. There is the vertical gap between the lead fingers and the die pad caused by the downset of die pad. The alternate process of forming the lead frame still provides the advantage of shortening the critical thermal path of the lead frame and therefore improve s the thermal dissipation characteristics of any plastic integrated circuit device package into which it is placed. However, the alternate embodiment has more process steps than does the preferred embodiment.
These and other objects of the invention will become apparent from the detailed description of the invention in which numerals used throughout the description correspond to those found in the drawing figures.